add time_range and time_region options to OcgOperations #210

Time and level subset options added to "OcgOperations" with docs.
These new parameters will override any values for time/level subsetting
on "RequestDataset" objects.

Closes #210.
Merge branch 'master' into i285-string-parsing

Conflicts:
	src/ocgis/api/interpreter.py
	src/ocgis/api/parms/definition.py
	src/ocgis/test/test_ocgis/test_api/test_operations.py